This application relates to semiconductor quantum well devices for modulating the intensity and/or phase of incident light by variation of applied voltages.
Much work has been done recently on a wide range of electro-optic devices based on the electric-field dependence of strong absorption resonances in semiconductor quantum wells (Qws). These devices typically manipulate light having photon energies near the bandgaps of the quantum wells, corresponding to wavelengths around 1000 nanometers (nm) for gallium arsenide (GaAs) and low-indium-concentration InGaAs.
In a QW, a layer of one semiconductor material is sandwiched between cladding layers of a different material, with the electronic properties of the materials being such that an electric potential well (in the central layer) is formed between two electric potential barriers (in the cladding layers). The QW's small thickness, on the order of 100 .ANG., results in quantization of charge-carrier motion in the thickness direction that leads to formation of electron and hole sub-bands in the conduction and valence bands, respectively.
In addition, in QWs pairs of electrons and holes form bound states, called excitons, that are stable at room temperature because their binding energies are larger in a QW than in a bulk semiconductor. The excitons give the QW an optical absorption spectrum that has two peak wavelengths, a longer-wavelength peak due to heavy-hole excitons that is generally narrower and more optically useful than a shorter-wavelength peak due to light-hole excitons.
The stability of excitons in a QW leads to the quantum-confined Stark effect, in which the wavelengths of the QW's peak optical absorptions associated with the creation of light- and heavy-hole excitons shift to longer wavelengths in response to an applied electric field. Since these peak excitonic absorptions have finite spectral widths due to electron/hole interactions with material impurities and phonons, the transmissivity of a QW at a wavelength near a peak varies as the applied field varies. Typical electric fields range from 10,000 to 100,000 volts per centimeter. The wavelength shifts in the peak excitonic absorptions also lead to a variation in the refractive index of the QW layer at wavelengths slightly longer than the peak wavelengths, making it possible to obtain sizable shifts in optical phase. These and other aspects of QW devices are described in commonly assigned U.S. Pat. No. 5,047,822 to Little, Jr., et al., which is expressly incorporated here by reference.
Because a single QW is so thin, devices are typically made by stacking a number, e.g., fifty, of QWs in order to obtain significant optical effects. Many aspects of multiple quantum well (MQW) devices are described in the literature, including C. Weisbuch et al., Quantum Semiconductor Structures, Academic Press, Inc., San Diego, Calif. (1991).
A simple MQW device is the absorption modulator, in which the excitonic absorption edge of the quantum wells is moved into and out of coincidence with the wavelength of a spectrally narrow light source, such as a laser, by varying an applied electric field. Thus, the intensity of the light transmitted or reflected by the modulator varies according to the applied electric field, or bias voltage, as noted above.
One such absorption modulator, although based on Wannier-Stark localization rather than the quantum-confined Stark effect, is described in K.-K. Law et al., "Normally-Off High-Contrast Asymmetric Fabry-Perot Reflection Modulator Using Wannier-Stark Localization in a Superlattice" Applied Physics Letters vol. 56, pp. 1886-1888 (May 7, 1990); and K.-K. Law et al., "Self-Electro-Optic Device Based on a Superlattice Asymmetric Fabry-Perot Modulator with an On/Off Ratio&gt;100:1", Applied Physics Letters vol. 57, pp. 1345-1347 (Sept. 24, 1990). In contrast to the QW's shift of the excitonic absorption peaks to longer wavelengths due to the quantum-confined Stark effect, Wannier-Stark localization leads to a shift to shorter wavelengths for increased electric field in superlattice structures.
Aspects of superlattices, which are also structures of thin alternating layers of two materials having different electronic properties, are described in G. Dohler, "Solid-State Superlattices" Sci. Am. vol 249, pp. 144-151 (Nov. 1983). In general, a superlattice is a stack of interleaved thin barrier layers and QWs in which the Qws are resonantly coupled, causing the QWs' discrete charge-carrier energy levels to broaden into minibands. Applying an electric field destroys the resonance, misaligning the energy levels in neighboring QWs and localizing them over a few Qws. This changes the optical absorption spectrum from a smooth, miniband profile to a peaked, Qw-excitonic profile and blueshifts the absorption edge.
As described in more detail below, Applicants' invention can be embodied using either MQW or superlattice structures. Also, it will be understood that such structures described in this application can be fabricated by a wide variety of semiconductor processing methods, e.g., metal-organic chemical vapor deposition, molecular beam epitaxy, and electrochemical deposition methods. See, e.g., J. Switzer et al., "Electrodeposited Ceramic Superlattices" Sci. vol 247, pp. 444-445 (Jan. 26, 1990); and the above-cited Weisbuch et al. book.
Other MQW modulators capable of varying the intensity and phase of optical signals are described in U.S. Pat. No. 4,727,341 to Nishi et al and T. Wood et al., "High-Speed Optical Modulation with GaAs/GaAlAs Quantum Wells in a P-I-N Diode Structure", Applied Physics Letters vol. 44, pp. 16-18 (Jan. 1, 1984).
Another type of MQW absorption modulator is described in U. Koren et al., "InGaAs/InP Multiple Quantum Well Waveguide Phase Modulator", Applied Physics Letters vol. 50, pp. 368-370 (Feb. 16, 1987). The device consists of sixty InGaAs well layers and indium phosphide (InP) barrier layers disposed in the undoped core of an optical waveguide doped as a P-I-N structure. A phase shift of 180.degree. for light having a wavelength near 1500 nm is reported for a bias of 15-20 volts.
The recent publication, V. Gorfinkle et al., "Rapid Modulation of Interband Optical Properties of Quantum Wells by Intersubband Absorption" Applied Physics Letters vol. 60, pp. 3141-3143 (Jun. 22, 1992), describes the theory of a doped MQW absorption modulator in which the band-to-band absorption strength for near-infrared (NIR) photons, i.e., wavelengths from about 800 nm to 2000 nm, would be modulated by intersubband absorption of long-infrared (LIR) photons, i.e., wavelengths from about 8000 nm to 12000 nm. The LIR absorption would partially deplete the population of carriers in the ground state, thereby changing the density of final states for NIR absorption. A significant drawback of such a device for a purpose such as converting LIR information into NIR information would be the interdependence of the operating LIR and NIR wavelengths due to the absorptions occurring in the same MQW structure. Moreover, a very large LIR flux and fabrication in a waveguide geometry are needed for significant NIR absorption modulation.
Simple MQW absorption modulators operating at room temperature can exhibit modulation depths, i.e., ratios of minimal to maximal absorptions, of about 10:1 to 30:1. These low modulation depths can be improved by combining an MQW structure with a suitable resonant optical cavity, such as an asymmetric Fabry-Perot etalon (ASFPE). An ASFPE is a resonant optical cavity formed by two planar mirrors that have different reflectivities. Like a symmetric FPE, the wavelengths at which the ASFPE resonates are periodically distributed and are determined by the distance between the mirrors. Design aspects of ASFPE-MQW modulators are described in K. -K. Law et al , . "Superlattice Surface-Normal Asymmetric Fabry-Perot Reflection Modulators: Optical Modulation and Switching" IEEE J. Quantum Electronics vol. 29, pp. 727-740 (Feb. 1993).
For a normally-on ASFPE-MQW modulator, i.e., a modulator having high transmission (or reflection) when no electric field is applied to the MQW structure, one of the Fabry-Perot resonance wavelengths is set to a wavelength slightly longer than the peak wavelength of the quantum wells' heavy-hole exciton absorption under zero voltage bias. Applying the proper bias voltage causes the absorption peak wavelength to shift toward the FPE-resonance wavelength, thereby balancing the effective reflectivities of the mirrors and modulating the light transmitted or reflected from the device.
An array of such ASFPE-MQW modulators can be disposed on a substrate, by epitaxial growth, for example, to form a spatial light modulator (SLM). A two-dimensional array of absorption modulators is described in T. Wood et al., "High-Speed 2.times.2 Electrically Driven Spatial Light Modulator Made with GaAs/AlGaAs Multiple Quantum Wells (MQWs)", Electronics Letters vol. 23, pp. 916-917 (Aug. 13, 1987). The modulators are planar devices formed on a GaAs substrate, consisting of fifty AlGaAs/GaAs quantum wells disposed in an undoped region of a reverse-biassed P-I-N diode and surrounded by thin undoped superlattice regions. Incident light propagates through the modulators and perpendicular to the substrate, yielding a reported on/off intensity ratio of about 1.5:1 for wavelengths near 850 nm.
U.S. Pat. No. 5,115,335 to Soref describes an array of MQW devices and ASFPEs for binary or ternary modulation of the phase of an input light beam. The simultaneous variation of amplitude and phase is also discussed, and an electrical bias on the MQW devices is varied to obtain the phase modulation. On the other hand, the bias on each MQW device of the array must be selectively adjusted to compensate for device thickness variations. Also, the Soref patent does not describe continuous modulation of either phase or amplitude.
U.S. Pat. No. 5,107,307 to Onose et al. describes an intensity modulator comprising an MQW device disposed in an FPE and a spatial modulator comprising an array of such MQW-FPE combinations. The transmissivity of the intensity modulator can be continuously selected by varying a bias voltage applied to the MQW device. Onose is silent on phase modulation and on any technique for compensating for the inevitable variations in the modulator characteristics.
U.S. Pat. No. 4,790,634 to Miller et al. describes an optically bistable FPE in which the mirrors may have differing reflectivities, i.e., the FPE may be asymmetric. Miller illustrates PIN-diode-type MQWs disposed within FPEs, and describes varying a bias voltage on the MQWs to tune the bistable structure. On the other hand, Miller is silent on any technique for compensating for the inevitable variations in the modulator characteristics.
U.S. Pat. No. 4,525,687 to Chemla et al. describes a light modulator that includes MQWs. The shift of the peak wavelength of heavy-hole exciton absorption is described, as are PIN diode structures. The MQWs may be disposed in FPEs, with the resulting structures being electrically tunable.
Many current ASFPE-MQW SLMs have the incident light strike the top of the ASFPE-MQW structures because the substrates are usually opaque to the incident light. Thus, the electronic circuits used to bias the quantum wells are connected via electrically conductive lines that must be located on the surfaces of the ASFPE-MQW structures. Connecting the optical sections (the ASFPE-MQW structures) to the electronic sections (the voltage bias circuits) of the SLM in this way limits the practical array size and speed. As an array grows larger, the multiple conductive lines occupy larger and larger portions of the optical sections' surfaces, reducing the optically useful portion of the substrate area. The length of the conductive lines and their proximity to one another also limit the speed of the device due to the impedance of and cross-talk between long lines.
Another drawback of previous ASFPE-MQW spatial light modulators is they cannot be easily modified after the optical sections have been deposited on the substrate. If the thickness of an FPE cavity does not produce a resonance wavelength that is correctly placed relative to the corresponding quantum wells' absorption peak wavelength, the thickness cannot be altered via standard semiconductor processing techniques. Moreover, variations in the layer thicknesses, inherent in standard deposition processes, cause variations in the FPE resonance wavelength across an array that cannot be easily corrected. Prior devices have attempted to compensate for these variations by applying individual correction voltages to each array element. This approach, however, affects the operating wavelength, greatly increases the complexity of the bias and control electronic circuits, and due to the increased size of the circuitry, limits the fill factors of the arrays to about 70% and drastically reduces their speed as a result of the increased capacitance.
Some prior modulators have attempted to overcome these problems by first epitaxially growing the ASFPE and MQW structures on a substrate like GaAs and then bonding a silicon substrate having desired electrical connections on top that fan out from the SLM array to electrical contact pads outside the area covered by the SLM array. Such a device is described in A. Moseley et al., "Uniform 8.times.8 Array InGaAs/InP Multiquantum Well Asymmetric Fabry-Perot Modulators for Flipchip Solder Bond Hybrid Optical Interconnect", Electronics Letters vol. 28, pp. 12-14 (Jan. 2, 1992). For large two-dimensional arrays, this type of electrical addressing is impractical.
Another such device is described in K. Hu et al., "Inverted Cavity GaAs/InGaAs Asymmetric Fabry-Perot Reflection Modulator" Applied Physics Letters vol. 59, pp. 1664-1666 (Sep. 30, 1991). The Hu paper also mentions that the as-grown MQW cavity can be characterized and its thickness adjusted prior to deposition of the second mirror of the ASFPE in order to tune the cavity's resonance wavelengths.
Other devices have had MQW structures grown on a GaAs substrate without ASFPE structures, then had another substrate like sapphire bonded on top, and then had at least part of the GaAs substrate etched away to allow optical access to a single pixel See I Bar-Joseph et al., "Room-Temperature Electroabsorption and Switching in a GaAs/AlGaAs Superlattice", Applied Physics Letters vol. 55, pp. 340-342 (Jul. 24, 1989).